This invention relates to an improved process for forming multilayer articles, such as, for example, tubes or parisons or other preforms and then blowmolding them into bottles or similar articles.
Plastic bottles and other containers are gaining increased acceptance in the packaging of such diverse articles as, for example, beverages, household articles, and motor oils. In certain applications, such bottles must satisfy a number of strict requirements, including sufficient rigidity and strength, sufficient barrier properties to prevent significant entry of oxygen or significant evaporation or leakage of liquids contained therein, and acceptable chemical composition of the innermost layer to assure absence of toxic components or contaminants when used in contact with food products.
Because of those requirements, commercial plastic containers such as bottles often are rather complex articles having multilayer walls, each layer being designed to supply the desired properties. Thus, there is at least one layer of plastic material which provides strength and/or rigidity and is generally referred to in the industry as the structural polymer layer. This may be, for example, polypropylene, polyethylene, polycarbonate, poly(ethylene terephthalate), polyamide, polystyrene, or polyvinyl chloride. It usually will be either the innermost or the outermost layer, or both. The barrier material usually will be one of the intermediate layers and normally will be laminated to its neighboring layers by means of adhesive layers. The barrier material frequently will be a polyamide, polyvinylidene chloride, or ethylene/vinyl alcohol copolymer. The adhesive layers may be, e.g, an ethylene/vinyl acetate copolymer, a low or high density polyethylene composition containing an ethylene/vinyl acetate and other copolymers, or a polypropylene composition containing a maleic anhydride component. At least one of the layers often is made totally of reground polymer or blended with virgin structural resin, the reground material being recovered by trimming the parison in the process of blowmolding articles therefrom. This helps reduce waste and lower the manufacturing cost. However, use of this reground material, which contains both the inexpensive structural polymer and the expensive barrier and adhesive resins, still does not solve the cost reduction problem in the most efficient manner. It would be desirable to be able to adjust the parison composition in such a manner that the material separating the segments from which individual blowmolded articles such as bottles or other containers are made consists to the extent possible only of the inexpensive structural polymer, so that substantially no barrier resin or bonding resin is removed with the parison trim.
An extruding apparatus for producing tubing having different physical or chemical characteristics along its axial length is known from U.S. Pat. No. 3,724,985 to Burlis et al. It comprises two or more devices feeding different molten plastic materials which devices are controlled such that when one is delivering material to the die producing the first section of the tubing, another one either is not delivering or is exhausting to an accumulator, the respective on and off cycles being controlled by an adjustable valve. In this way, each device reinjects from time to time into the die material previously withdrawn from the apparatus.
Japanese Laid Open Patent Application (Kokai) No. 53/120773 discloses a multilayer molding process, applicable especially to the extrusion of articles having layers of different colors, wherein different resins are introduced into a multilayer extrusion molding head and are extruded either simultaneously or intermittently, the flow of each resin being separately controlled by means of a valve.